The Market Bulletin, March 2009 IssueSharka (or plum pox) is a viral disease caused by the plum pox virus (PPV), which can infect most ornamental and agriculturally significant species in the genus Prunus, including fruit trees such as plums, peaches, and apricots. Ornamental species of Prunus may show few symptoms of plum pox virus but can be carriers of PPV. In commercial stone fruit orchards the disease blemishes fruit, reduces yield and quality and is considered the most devastating disease of stone fruit in Europe.

The disease was first described in Bulgaria in the early 1900s and has since spread throughout Europe. PPV was first reported in North America in Adams County, Pennsylvania in 1999 and in Ontario and Nova Scotia in 2000. In the U.S. control efforts were thought to have eradicated the disease, but during 2006 surveys, the virus was found again on commercial plums and peaches in Niagara County, New York, on commercial peaches in Pennsylvania and in a plum research plot in southwestern Michigan. Additional surveys conducted in 2007 and 2008 have since detected PPV in three New York counties and 9 additional properties in Ontario, all within the greater Niagara region.

The source of PPV in North America is still unknown. The presence of multiple strains of PPV and a diversity of PPV D-Strain genotypes in the greater Niagara region suggest several introduction events have occurred in the U.S. and Canada. PPV D-Strain populations found in Pennsylvania are distinct from the viral populations found in New York and Canada suggesting a separate introduction event in Pennsylvania as well.

PPV is spread over long distances in Prunus seedlings and root stock as well as budwood. Seeds and fruit are not thought to be significant carriers of the disease. Once established in an orchard, aphids transfer the virus for shorter distances between trees or between adjacent orchards. In Canada, surveys have found PPV in Prunus species at several small residential hobby farms. The genetic diversity of viral isolates found on hobby farms in Ontario is generally higher than the diversity found in commercial orchard settings, suggesting homeowner plantings may have been the primary source of PPV in Ontario. This may be the case in the U.S. as well but fewer isolates have been recovered from smaller home orchards and less data is available. The hypothesis that hobbyists have contributed to the introduction of PPV in North America has gained traction among many researchers and growers since regulations have been in place for years to keep stock used for commercial planting disease free through clean stock programs.

In order to protect the health of stone fruit crops in West Virginia and the region, in 2009 the West Virginia Department of Agriculture (WVDA) will be conducting a survey for PPV on peaches and other ornamental Prunus species within the state. The survey will focus on Berkeley, Jefferson and Hardy counties. To a lesser extent trees will be sampled for PPV in counties along the Ohio River. Both commercial and ornamental home and hobbyist plantings in the vicinity of commercial orchards will be targeted. The sampling procedure for PPV is not harmful to the trees sampled. The protocol calls for collecting 5-6 leaves from the base of the last year’s growth as this has been found to contain the highest amount of PPV. Leaves from at least four branches are collected from opposite sides of each tree since the virus often infects sectors of the tree. The leaves will then be brought to the Gus R. Douglass Agricultural Station at Guthrie for processing and ELISA testing. Finding PPV in an orchard at early stages of infection greatly benefits the grower and neighboring growers by slowing the progression of this potentially devastating disease. Since fruit is not known to be a carrier, it can still be harvested and moved off site from orchards with infected trees. Current federal guidelines require that all infected trees be removed to eradicate the pathogen. In most situations in North America only a small fraction of trees have had to be removed from orchards and growers have been compensated for these trees.

The WVDA is requesting the help of peach growers and home owners with hobby orchards containing Prunus species in determining the presence or absence of PPV in the state by agreeing to let the WVDA survey private orchards this spring. Please let the WVDA know if you are interested in participating in the survey by calling (304-558-2212) or emailing (ndart@ag.state.wv.us) by March 31, 2009. In order to participate we need: (1) your name, (2) the street address of your nursery or home planting and (3) a phone number where you can be reached.